The telecommunications industry has been working with polarization adjusters, controllers, and scramblers for many years. Typical uses include: optimizing optical power transmission through polarization dependent components; detecting polarization dependence of a component (by monitoring optical power at the output of the device while methodically scanning all the polarization states at the input of the component); and curtailing polarization dependence of components and detectors (by scanning substantially all possible states of polarization and illuminating the components and the detector at a rate faster than the signal sampling rate). Polarization controllers having endless control capabilities, which allow for reset free operation, are also in use in polarization mode dispersion compensators. Heismann teaches such an endless polarization controller in U.S. Pat. No. 5,212,743 issued May 18, 1993.
Various polarization control mechanisms employed by polarization controllers include rotatable fiber coils, fiber squeezers, variable orientation retarders, electro-optical waveguides and bulk devices with controllable retardance and optical axis orientation, and variable retardance retarders such as liquid crystal cells (LCC's). Polarization controllers based on LCC's are taught by Clark et al. in U.S. Pat. No. 5,005,952 issued Apr. 9, 1991, which discloses a stack of multiple liquid crystal cells. Rumbaugh et al. also teach a polarization controller in U.S. Pat. No. 4,979,235 issued Dec. 18, 1990, based on a stack of three liquid crystal cells.
LCC-based controllers are often used in feedback loops, in which they change the state of polarization in response to a signal. LCC-based controllers are designed to be able to transform any input state of polarization into any output state of polarization.
In the fields of test instrumentation and laboratory equipment, waveplate-based polarization controllers comprise a linear polarizer, a quarter waveplate and a half waveplate. A commercially available version of this polarization controller is manufactured by the JDS Uniphase Corporation under the PR2000™ product name. Rotatable fiber coils, taught by LeFevre in U.S. Pat. No. 4,389,090 issued Jun. 21, 1983, are also often used, but are cumbersome and do not always provide adequate repeatability. Furthermore, the slow response time and wavelength dependency of these polarization controllers require the user to schedule long duration tests and to make wavelength corrective approximations. Solid state opto-electrical waveguide devices are faster but are prone to drift problems. Fiber squeezers do not provide very reproducible results.
There is also a need in the field of test instrumentation for rapidly and reproducibly generating a fixed number of polarization states. This is a relatively easy task when the required states of polarization are linear. In this case a polarizer or a polarization prism can be mounted in a precise, motor-controlled goniometer. A specific application that could use a rapid and reproducible way to generate a fixed number of polarization states is the Mueller method for polarization dependent loss (PDL) measurement as described in the IEC document number CEI/IEC 61300-3-12:1997 incorporated herein by reference. This method requires the generation of four states of polarization with no more than three of the four states lying in a common plane in the Poincaré sphere representation. A common method of generating in a time sequence the four required states is to have a light source illuminate a circular polarizer providing circularly polarized light, and then to have three linear polarizers (0°, 45°, and 90°) sequentially disposed in the path of the circularly polarized light. One can also use the same polarizer mounted in a motor-controlled goniometer. Thus, one state of circularly polarized light and three states of linearly polarized light can be generated, and the PDL of a component measured. This mechanical toggling of the states of polarization is time consuming. A commercially available instrument based on the Mueller method for measuring PDL is manufactured by the JDS Uniphase Corporation under the PS3™ product name and requires approximately two seconds to complete one measurement.
Thus, there is a need for a polarization-controlling device that allows a relatively quick variation of the state of polarization of a light beam in a predictable and reproducible manner. Furthermore, for polarization controllers destined for the laboratory or the test bench, it may not be necessary to be able to transform an arbitrary input state of polarization into an arbitrary output state of polarization.